This invention relates to a glass electrolyte composition for use in high-temperature electrochemical cells and batteries of such cells. It is contemplated that these batteries will have automotive and other vehicular use as well as use in the storage of electrical energy such as for off peak power load shifting.
Sodium-sulfur electrochemical cells are most often provided with an electrolyte of crystalline beta alumina which is characterized by very high ionic conductivity, for instance in the range of 0.1 to 1.0 (ohm-cm).sup.-1. However this material like other crystalline compositions is frangible, brittle, and requires difficult and expensive fabrication techniques. Furthermore, it has not endured attack particularly at grain boundaries on long exposure to molten sodium, sulfur and polysulfides at cell conditions.
Amorphous glass electrolytes, although having greatly improved fabrication properties, have exhibited low ionic conductivities and often poor stability in the cell environment. Glass compositions predicted to have improved ionic conductivity may be difficult or impossible to prepare as a clear glass free of crystalline regions or of undissolved constituents, thus severely degrading the chemical stability and good fabrication properties of the pure material.
In one program, work done under U.S. DOE Contract No. DE-AC02-76ET25103, a hollow glass fiber of Na.sub.2 O, B.sub.2 O.sub.3, NaCl and SiO.sub.2 was developed as electrolyte for a sodium-sulfur, secondary electrochemical cell. Very small diameter, elongated fibers provided a large electrolyte surface area and short ion migration path to at least partially offset the low ionic conductivity, e.g. at 300.degree. C., 4-5.times.10.sup.-5 (ohm-cm).sup.-1 of the glass.
A substantial improvement in ion conductivity is exhibited by the glasses of Susman et al, U.S. Pat. No. 4,465,744. Ionic conductivities at 300.degree. C. of up to 1.93.times.10.sup.-3 (ohm-cm).sup.-1 were found in glasses having compositions by mole percent of 29.6 to 33.3% Na.sub.2 O, 16.7 to 22.7% ZrO.sub.2, 0 to 6.8% P.sub.2 O.sub.5, and 40.9 to 50% SiO.sub.2. Neither this patent nor applicants' subsequent research effort reveal glasses of single phase in this composition range with ionic conductivities in excess of 2.times.10.sup.-3 (ohm-cm).sup.-1.
Many of the prior compositions are difficult to prepare as a glass substantially free of crystalline regions or free phases of undissolved constituents. Special procedures and conditions, for instance as are described in U.S. Pat. Nos. 4,465,744 and 4,432,891, may be required. Where glass transition temperatures are low, the quenching must be conducted over a large temperature span from well above the melting point, rapidly through crystal nucleation temperatures to well below the glass transition temperature. Glasses with low transition temperatures may not remain stable at the high operating temperatures, of for instance a sodium-sulfur electrochemical cell. Also regions of undissolved constituents may be subject to chemical attack by the cell reactants.
Consequently, it was unclear whether glass compositions outside the explored ranges could be prepared with substantially improved ionic conductivities. Also, it was unknown whether other compositions would be stable and chemically resistant at cell operating conditions.
Therefore in view of the foregoing it is an object of the present invention to provide an ionically conductive glass with improved ionic conductivity over those previously available for use in high-temperature, sodium-sulfur electrochemical cells.
It is a further object to provide such a glass substantially free of crystalline regions and undissolved constituent phases.
It is also an object to provide such a glass that is sufficiently stable and chemically resistant to high-temperature, sodium-sulfur environment such that the ionically conductive glass may be employed as a solid electrolyte.